In the manufacture of motor vehicles and particularly vehicles powered by motors burning liquid fuel such as gasoline, it has recently been required to provide an on-board diagnostic system for detecting malfunction or leakage of the fuel vapor emission control system on the vehicle.
This requirement for on-board diagnostics of the fuel vapor emission system has resulted in the need to detect fuel vapor leaks in the fuel tank, vapor storage canister and the canister purge valve and venting conduits connecting the fuel tank with the storage canister, in addition to the purge control valve connecting the canister to the engine inlet for purging fuel vapors from the storage canister to the engine during operation.
With the advent of electrically controlled fuel injectors for motor vehicle engines, it has been possible to provide electronic management of combustion in the engine with an electronic controller, which includes electrically controlling the feed of fuel vapor from a storage canister into the engine during operation. Thus, an electrically operated purge control valve on the canister has been employed.
Furthermore, requirements for minimizing the escape of fuel vapors during refueling operations; and, requirements that the vent for the vapors from the tank to the canister be closed in the event of vehicle rollover, have all created a need for valving arrangements which must be installed through openings either in the tank or the filler neck or in the lines from the tank to the storage canister. In addition, the advent of electric fuel injectors has resulted in electrically operated fuel pumps installed through an access opening in the fuel tank. Thus, the complexity of the fuel supply and vapor emission control system has resulted in numerous connections and seals which may become subject to leakage in prolonged service due to vibration and deterioration of the materials employed. Leakage has been a problem particularly where plastic materials are used which may be subject to absorption and swelling after prolonged exposure to the fuel.
Thus, it has been required to provide a way of detecting the failure of the vapor integrity of the fuel and vapor vent system or, in other words, provide the ability to detect leakage in any of the components employed in the fuel/fuel vapor emission control system. Leakage of fuel vapor under positive pressurization can also occur when the ambient conditions reach elevated temperatures particularly as may result from proximity of the fuel tank to the engine exhaust system. Leakage of atmospheric air into the fuel system can also occur as the system may be subjected to sub-atmospheric pressure resulting from cooling after prolonged usage in warm climate conditions and particularly in such conditions where the tank is normally warmed by proximity to the engine exhaust system.
Thus, it has been desired to provide a simple, relatively low cost way of detecting the presence of vapor leakage in the vehicle fuel/fuel vapor emission control system and to provide an indication of leakage in order that the vehicle operator may have the faulty condition serviced.